Radio frequency properties of ceramic, high-Tc superconductors

The impedance of circuits consisting of superconducting ceramic strips (typically 58 mm long, 5.4 mm wide, and 0.175 mm thick), with wires indium-soldered to them, was measured and compared with that of copper strips of the same size and with similar connections. The superconducting strips were encapsulated in glass to stop deterioration due to moisture. The circuits contained both internal and external inductance and resistance at radio frequencies. From Q measurements both the resistive and reactive components were calculated. The improvement factor for the circuit consisting of the copper strip cooled to liquid nitrogen temperature is 35% over the range from 1 MHz to 20 MHz. Comparing the circuit with the copper strip at room temperature with the superconductor at liquid nitrogen temperature the improvement factor is 63% at 1 MHz, decreasing to 56% at 20 MHz. This improvement may be worthwhile for some applications. Data were also taken at 1000 MHz and 10000 MHz. In both cases the improvement over copper was slight. These factors are quite sample-dependent but are always rather small for the ceramic material used. A theoretical discussion of the conductivity of these ceramics is included. The rationale for using indium solder is also examined.<>